An anatomic evaluation of L5 nerve stretch in spondylolisthesis reduction.

STUDY DESIGN:
Lumbosacral spondylolisthesis was simulated using four embalmed human spines, and the path of the L5 nerve was studied.

OBJECTIVES:
To quantify the change in length of the L5 nerve root associated with reduction of spondylolisthesis, correction of slip angle, and changing disc height.

SUMMARY OF BACKGROUND DATA:
Stretch injury to the lumbar nerves remains a complication of spondylolisthesis reduction. To date, no anatomic studies have been performed to quantify this effect of reduction on the lumbar nerves.

METHODS:
The L5 vertebral body and the sacrum of four embalmed human spines were constrained in an adjustable jig, and the length of a simulated nerve was determined for various position variables--sagittal translation (0-100% slip), slip angle (-40 degrees to +20 degrees), and disc height (5 or 10 mm). Two standard points of reference were chosen to represent fixed points along the path of the L5 nerve. An inelastic cord was used to measure the path length between these points as L5 was reduced from 100% to 0% slip. Testing was performed using a 5-mm and a 10-mm disc height. The effect of varying slip angle alone was also studied.

RESULTS:
The effect of spondylolisthesis reduction and slip angle correction on nerve length varied depending on the location of L5 with respect to the sacrum. There was an increasing effect of partial reduction on nerve length as L5 approached full reduction. Initially, little strain was produced in the L5 nerve as L5 was reduced in higher grade slips. However, as L5 approached full reduction, the strain per increment of reduction increased rapidly. On average, the mean nerve strain was 4.0% for the first 50% of reduction and 10.0% for the second half of reduction. Increasing lordosis relaxed the nerve in high-grade slips and stretched the nerve in fully reduced slips. At 100% slip, the mean nerve excursion decreased 5.1 mm (nerve slackening) when L5 was rotated from +20 degrees to -40 degrees. At 0% slip, the mean nerve excursion increased 3.1 mm (nerve stretch). Increasing disc height directly stretched the L5 nerve. However, given a larger disc height, the strain on the nerve per increment of reduction was less than for the smaller height.

CONCLUSION:
The findings suggest that the risk of stretch injury to the L5 nerve with reduction of a high-grade spondylolisthesis is not linear; with 71% of the total L5 nerve strain occurring during the second half of reduction, partial reduction may be a significantly safer treatment approach for high-grade spondylolisthesis than complete reduction. Correction of lumbosacral kyphosis in high-grade spondylolisthesis may be protective of the L5 nerve.